PROJECT SUMMARY Our published work and preliminary data using mouse models and in vitro cell cultures show that extracellular matrix (ECM) molecules such as versican accumulate in lungs following exposure to viruses and bacteria. The creation of a specialized versican-enriched ECM coincides with invasion and retention of leukocytes in lungs during the immune response to influenza virus. We have found that two cellular sources synthesize versican in response to Toll-like receptor (TLR) agonists: macrophages and fibroblasts. Studies identify two distinct signaling pathways directing versican synthesis in these two cell types - the ?-catenin/T-cell factor (TCF) pathway in stromal cells such as smooth muscle cells and fibroblasts and the TLR/Trif/Type I interferon (IFN) receptor pathway in macrophages. However, the cellular specificity of these signaling pathways is not known. Using two novel strains of genetically engineered mice in which the versican gene is deleted in a cell- and time-specific manner, our published and preliminary data shows that versican has a dramatic effect on leukocyte phenotype. When treated with polyinosinic:polycytidylic acid (poly(I:C)), mice with a global deficiency in versican have decreased recovery of leukocytes in bronchoalveolar (BAL) fluid suggesting that versican is pro-inflammatory and required for leukocyte migration into lungs. In contrast, mice lacking versican in macrophages have increased recovery of leukocytes in BAL fluid suggesting that macrophage-derived versican is anti- inflammatory and suppresses leukocyte migration into lungs. Both strains of versican-deficient mice had significantly decreased recovery of Type I IFNs and IL-10 in lung tissue or BAL fluid when compared to controls. Our in vitro studies attribute this to decreased production of these anti-inflammatory cytokines by versican-deficient macrophages. The differences observed in these novel versican-deficient mice are the basis of our central hypothesis that the formation of a versican-enriched ECM by macrophages and stromal cells provides critical contextual cues and extracellular-control of the innate immune response to viral lung infection. We propose three aims to test this hypothesis. Aim 1 defines the role of versican derived from macrophages and/or Type I IFN signaling in providing extracellular-control of the innate immune response in lungs of mice exposed to influenza virus. Aim 2 determines if stromal cells and/or ?- catenin/TCF signaling promote the generation of a versican-enriched ECM that provides fine-control of the innate immune response to influenza virus. Aim 3 identifies the mechanisms and signaling pathways whereby macrophage- versus fibroblast-derived versican provide contextual extracellular-control of the innate immune response. Understanding the contextual settings in which versican provides fine-control of the innate immune response to influenza virus is critical for the design of therapeutic strategies for improving the immune response to viral lung infection.